The largest liquid-argon neutrino detector in the world has just recorded its first particle tracks, signaling the start of a new chapter in the story of the international Deep Underground Neutrino Experiment (DUNE).

DUNE’s scientific mission is dedicated to unlocking the mysteries of neutrinos, the most abundant (and most mysterious) matter particles in the universe. Neutrinos are all around us, but we know very little about them. Scientists on the DUNE collaboration think that neutrinos may help answer one of the most pressing questions in physics: why we live in a universe dominated by matter. In other words, why we are here at all.

The project includes a substantial William & Mary contingent, supported by the National Science Foundation. The team includes Jeff Nelson and Mike Kordosky, both of whom are professors in the university’s Department of Physics. Two postdoctoral researchers, Alex Radovic and Edgar Valencia, worked on ProtoDUNE, as did graduate students Lauren Liegey and Luis Zazueta Reyes.

Four William & Mary undergraduates also worked on ProtoDUNE, including a stint at CERN in 2017. Will Henninger, machine shop manager for the physics department, is another vital member of William & Mary’s ProtoDUNE crew.

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The enormous ProtoDUNE detector — the size of a three-story house and the shape of a gigantic cube — was built at CERN, the European Laboratory for Particle Physics, as the first of two prototypes for what will be a much, much larger detector for the DUNE project, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory in the United States. When the first DUNE detector modules record data in 2026, they will each be 20 times larger than these prototypes.

It is the first time CERN is investing in infrastructure and detector development for a particle physics project in the United States.

Nelson explained that DUNE is a truly international project in neutrino physics and represents an expansion of involvement by CERN, the European consortium that’s the home of the Large Hadron Collider and site of the discovery of the Higgs boson.

“CERN is expanding beyond the boundaries of experiments hosted in its member states in a similar way that the U.S. is delivering major subsystems and magnets for the accelerators at CERN,” Nelson said. “One of the ways that CERN has contributed is what’s called the neutrino platform. There are a number of aspects to it, but one of the biggest is to provide a full functional tests of the DUNE far detectors. ”

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The first ProtoDUNE detector took two years to build and eight weeks to fill with 800 tons of liquid argon, which needs to be kept at temperatures below -184 degrees Celsius (-300 degrees Fahrenheit). The detector records traces of particles in that argon, from both cosmic rays and a beam created at CERN’s accelerator complex. Now that the first tracks have been seen, scientists will operate the detector over the next several months to test the technology in depth.

Valencia’s contribution to ProtoDUNE is the design, calibration and installation of the camera and lighting system in the detector, giving scientists a way to keep an eye on the argon.

“The main goal was to have a reliable camera and lighting system working inside the ProtoDUNE cryostat,” Valencia explained. “The idea is to check the liquid argon level during the filling process, check the formation of some bubbles and ice pieces and also identify possible high-voltage sparks or arcs.”

“Only two years ago we completed the new building at CERN to house two large-scale prototype detectors that form the building blocks for DUNE,” said Marzio Nessi, head of the Neutrino Platform at CERN. “Now we have the first detector taking beautiful data, and the second detector, which uses a different approach to liquid-argon technology, will be online in a few months.”

The technology of the first ProtoDUNE detector will be the same to be used for the first of the DUNE detector modules in the United States, which will be built a mile underground at the Sanford Underground Research Facility in South Dakota.

More than 1,000 scientists and engineers from 32 countries spanning five continents – Africa, Asia, Europe, North America and South America – are working on the development, design and construction of the DUNE detectors. The groundbreaking ceremony for the caverns that will house the experiment was held in July of 2017.

Nelson said the William & Mary involvement in the DUNE experiment goes back even before ProtoDUNE. “I guess it’s proto-ProtoDUNE,” he says, explaining that components for smaller, precursor detectors were assembled at William & Mary in Small Hall, the university’s physics building and then installed at Fermilab.

“Seeing the first particle tracks is a major success for the entire DUNE collaboration,” said DUNE co-spokesperson Stefan Soldner-Rembold of the University of Manchester, UK. “DUNE is the largest collaboration of scientists working on neutrino research in the world, with the intention of creating a cutting-edge experiment that could change the way we see the universe.”

When neutrinos enter the detectors and smash into the argon nuclei, they produce charged particles. Those particles leave ionization traces in the liquid, which can be seen by sophisticated tracking systems able to create three-dimensional pictures of otherwise invisible subatomic processes.

“CERN is proud of the success of the Neutrino Platform and enthusiastic about being a partner in DUNE, together with Institutions and Universities from its Member States and beyond” said Fabiola Gianotti, director-general of CERN. “These first results from ProtoDUNE are a nice example of what can be achieved when laboratories across the world collaborate. Research with DUNE is complementary to research carried out by the LHC and other experiments at CERN; together they hold great potential to answer some of the outstanding questions in particle physics today.”

DUNE will not only study neutrinos, but their antimatter counterparts as well. Scientists will look for differences in behavior between neutrinos and antineutrinos, which could give us clues as to why the visible universe is dominated by matter. DUNE will also watch for neutrinos produced when a star explodes, which could reveal the formation of neutron stars and black holes, and will investigate whether protons live forever or eventually decay. Observing proton decay would bring us closer to fulfilling Einstein’s dream of a grand unified theory.

“DUNE is the future of neutrino research,” said Fermilab Director Nigel Lockyer. “Fermilab is excited to host an international experiment with such vast potential for new discoveries, and to continue our long partnership with CERN, both on the DUNE project and on the Large Hadron Collider.”